Transformation of Covalent Organic Frameworks from N‐Acylhydrazone to Oxadiazole Linkages for Smooth Electron Transfer in Photocatalysis

Abstract

Covalent organic frameworks (COFs) are regarded as new platforms for solar-to-chemical energy conversion due to their tailor-made functions and pre-designable structures. Their intrinsic reversibility and the high polarization of organic linkages inevitably result in poor chemical stability and weak optoelectronic properties. Herein, one N-acylhydrazone-linked COF (H-COF) was converted into a stable and π-conjugated oxadiazole-linked COF via post-oxidative cyclization. Both chemical stability and π-electron delocalization throughout the reticular framework are significantly improved, leading to a high hydrogen evolution rate of 2615 μmol g À 1 h À 1 upon visible light irradiation, which is over four times higher than that of H-COF. This work provides a facile protocol for the fabrication of π-conjugated COFs and the modulation of photophysical properties for photocatalytic application.

Developing highly reducing conjugated porous polymer: a metal-free and recyclable approach with superior performance for pinacol C–C coupling under visible light

“…Notably, the strong peak at ∼3250 cm −1 corresponding to the N–H stretching vibration disappeared in TX-OXD-P, thereby supporting the formation of oxadiazole moieties. In the solid-state 13 C NMR spectrum, the chemical shifts at 165 ppm, 125–154 ppm, 56 ppm, and 13–36 ppm can be ascribed to the carbon of OXD, 11 aromatic rings, benzylic carbon, and alkyl chains, respectively (Fig. 1c).…”

2D Covalent Organic Frameworks Toward Efficient Photocatalytic Hydrogen Evolution

“…Furthermore, the hydrazone‐linked COFs could be further functionalized with thioether, which exhibited remarkable photocatalytic hydrogen evolution ability even in seawater [10a] . Hydrazone‐linked COFs were also converted into oxadiazole‐linked COFs via oxidization reaction, which endow COFs with narrow bandgaps and continuous π‐electron delocalization for high photocatalytic activity (2615 μmol h −1 g −1 ) [10b] …”

Tuning Acceptor Length in Photocatalytic Donor‐Acceptor Conjugated Polymers for Efficient Solar‐to‐Hydrogen Energy Conversion

“…Therefore, organic semiconductor photocatalysts have attracted remarkable attention in the past decade. And a variety of organic photocatalysts have been developed, including graphitic carbon nitrides ( g ‐C 3 N 4 ), [ 10‐18 ] covalent organic frameworks (COFs), [ 19‐25 ] conjugated microporous polymers (CMPs), [ 26‐36 ] covalent triazine‐based frameworks (CTFs) [ 37‐43 ] and linear conjugated polymers (LCPs). [ 44‐52 ] Although some of them can achieve hydrogen evolution rate (HER) exceeding 10 mmol·g –1 ·h –1 , [ 22,30‐32,40,47‐49 ] the solar‐to‐hydrogen energy (STH) conversion efficiency is still very low, far from the basic requirement of 10% for their practical application.…”